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The most attractive features of nanomaterials including their small size, large surface area, and reactivity might also be the main factors for their toxicity. Pulmonary bronchial epithelial cells are a potential target for toxicity during respiratory exposure. The tumor cell line A549, a pulmonary human type-II like epithelial cells, is widely used for evaluation of particle toxicity. We hypothesized that A549 cells are more resistant to toxicity compared to the normal epithelial cells. To test this hypothesis we examined the dose-dependent cytotoxicity effects of single-walled (SWCNT) and multi-walled (MWCNT) carbon nanotubes on NHBE and A549 cells using lactate dehydrogenase (LDH) release and the cell proliferation/viability WST-1 assays. We found that NHBE exhibited marked cytotoxicity, similar to that observed with hydrogen peroxide treatment (positive control), consistently by 24 hours post-exposure to high concentrations [50-150mu g/106 cells (1.5-4.5mu g/ml)] of CNT. In comparison, exposure of A549 cells to similar concentration of CNT resulted in only a milder response. Overall, the results presented here indicate that SWCNT or MWCNT induce cytotoxicity in human lung epithelial cells in a time and dose-dependent manner. Furthermore, we demonstrate a large variation between the particle-induced cytotoxic responses in A549 cells and normal primary epithelial cells.